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A young woman travels across a future American crippled by civil war looking for her lost family.

It’s a matter of perverse national pride that nobody does a dreary post-apocalypse like the British.

Dependably grim, where Americans get drunk on the spirit of fierce resistance when the bombs drop, Brits just grumble into their bottled water and count their cancers.

Into this nation of When The Wind Blows and Threads, comes Last King Of Scotland’s Kevin Mcdonald with an adaptation of Meg Rosoff’s 2004 YA hit How I Live Now, which follows hostile American transplant Daisy (Byzantium’s Saoirse Ronan) shuttled off to the English countryside to stay with her twee English cousins.

It’s an effective set up. Like Threadsit keeps Daisy and her insecurities in the foreground, while in the background news reports fret over peace conferences and Typhoons sonic-boom overhead. Daisy softens to her extended family, helped no end by her incestuous crush on handsome cousin Edmund (Defiance’s George MacKay) and a whistle-stop tour of Can’t We Have More Ginger Beer, Papa? children’s adventure cliches.

We don’t see the bomb drop, but the sudden gust of wind that interrupts the cast’s picnic is followed by a rain of ash and the idyll begins to unravel – first comes an weary embassy staffer offering Daisy a ticket home, then British soldiers forcibly evacuate and separate the family, the girls taken to a sterile gated community where Daisy resolves to make her way back to the farm and be reunited with the love of her life who also happens to be the son of her mother’s sister.

That nobody addresses the appropriateness of this relationship is the first sign of How I Live Now’s wildly osculating tone which careens from laudable frankness to Famous Five-style silliness with little warning.

Successful YA movies are successful because they don’t talk down to their audience. They realise that teenagers are balancing adult concerns with those transitional ones that are unique to adolescence – learning who you are versus who you want to be, fitting in or standing out to find a place in the world.

Daisy’s journey through this metaphorical landscape is fully engaging, echoed by the physical journey she’s forced to undergo. The moments of stark realism – unzipping bodybags to identify bodies, and witnessing an implied rape that she chooses not to prevent – lend it a real power. All of which makes the moments of pure Jilly Cooper – all soft focus hayloft sex scenes – and Enid Blyton – seriously, have you ever seen a child in a Native American headdress in the 21st Century? – absolutely baffling and totally gross.

The autotune of directors, Mcdonald lends himself well to this sort of tonal dissonance – leaning into the varying moods like a yachtsman with flat light and grey/blue filters for the work camp community and glittering sunshine through the gently swaying trees when they make it home.

In that respect, How I Live Now is surely the best executed awkward film you’ll ever see – like The Road directed by Chris Columbus instead of Jonathan Hillcoat.